Burner function in combusting process plants is monitored with the help of combustion chamber cameras. The combustion process in the combustion chamber is examined by pushing the tip of the camera's optical assembly into the combustion chamber. Debris accumulation on the front lens of the camera's optical assembly can be a serious drawback limiting the availability of the camera. In difficult conditions the front lens must be cleaned daily.
The GB patent application 2,127,174 describes a periscope-type assembly for monitoring pressurized combustion chambers. The optical system employed is not an arrangement suited to form an image for a camera, but rather, an assembly adapted to relay the image onto the retina of an eye. In such an embodiment the lens system is cooled with an external air flow blown into the chamber via a relatively large opening in the chamber wall in order to prevent lens contamination. In such an arrangement, however, the holes in front of the lenses are inconveniently large and axially misplaced for the desired end result. On one hand the holes are a limiting factor for the field of view and on the other hand, the wide diameter of the hole allows debris to adhere to the lens.
The U.S. patent publication 4,432,286 describes a solution based on the pinhole camera principle for preventing contamination. In such an arrangement the lens assembly is employed for moving the image plane farther away from the combustion chamber. The image plane is situated in the rearmost focal plane of the optical system, so the pinhole performs as the imaging element and the lenses cannot form the image of the object from any distance of the object in the image/imaging plane without presence of the hole. Consequently, the lens optics employed is not an image-forming optical system. The optical resolution of such an arrangement is determined by the diameter of the pinhole. If the image sharpness is desired to be improved the pinhole diameter must be reduced. A smaller diameter of the pinhole leads to a reduced relative aperture. In the embodiment described in the cited publication, the field of view is narrow, approx. 30.degree..